The invention relates to the field of parametric amplifiers, and in particular to micro-fabricated parametric amplifiers.
An ideal differential voltage amplifier is often characterized by low-input bias currents, high input impedance, low input-referred voltage offset and drift, low input-referred voltage and current noise, wide bandwidth response, wide input common-mode swing, high common-mode rejection, and either high differential gain for operational amplifiers or stable differential gain for instrumentation amplifiers. For many potential applications, e.g. pH meters, thermocouples, electrometers, or ion gauges, wide bandwidth is not particularly important, while the demands on the amplifier with regard to noise, input impedance, leakage currents, offset/drift and common mode range are quite high. For applications of this sort, the amplifier""s mechanism of input signal transduction is of utmost importance since it will define the performance of the amplifier with regard to these metrics.
Current solid-state microfabrication technology limits the performance of amplifier topologies. The potential solid-state transducers that are in common use today include the bipolar junction transistor (BJT), the metal-oxide-semiconductor field effect transistor (MOSFET), and the junction field-effect transistor (JFET). The BJT exhibits excellent offset and noise performance, but at the expense of large input bias currents and low input impedance. The MOSFET transistor provides excellent low-leakage bias currents and generally high impedances, but compromises amplifier performance due to large offsets, drift, and noise. The JFET often strikes a reasonable balance between low leakage currents, noise, and offsets, but leakage performance severely degrades at elevated temperatures where many instrumentation applications operate. As a final,consideration, circuit topologies that utilize these solid state devices suffer from severe limits in their common-mode swing. The principles of differential voltage amplifiers are further discussed in xe2x80x9cA 50-fA Junction-Isolated Operational Amplifier,xe2x80x9d IEEE Journal of Solid-State Circuits, vol. 23, no 3, pp843-851, June 1988; xe2x80x9cFundamentals of Low-Noise Analog Circuit Design,xe2x80x9d Proceedings of the IEEE, vol. 82, no. 10, pp1514-1538, October 1994 and xe2x80x9cWith Input Bias Current of 40 fA, OP AMP IC Makes Low-Level Measurements,xe2x80x9d Electronic Design Exclusive.
The offset and drift performance of low-leakage amplifiers (e.g., MOSFET) can be enhanced with signal processing techniques. For example, the input signal can be modulated to an AC frequency beyond the 1/f noise corner of the input transducer and thereby amplified within a spectral region of relatively low noise. The amplified AC signal is then demodulated and filtered, yielding relatively low offset and drift amplification with MOSFET input transistors; this technique is commonly referred to as chopper-stabilization. A more detailed description of chopper stabilization techniques is provided in xe2x80x9cCircuit Techniques for Reducing the Effects of Op-Amp Imperfections: Autozeroing, Correlated Double Sampling, and Chopper Stabilization,xe2x80x9d Proceedings of the IEE, vol. 84, no. 11, pp1584-1614, November 1996.
Many signals of interest, however, are inherently DC or low frequency and provide no mechanism to modulate their spectral characteristics to a higher AC frequency. In these applications, additional circuitry or methods are needed to provide the required modulation. Many techniques exist which either chop the signal input""s polarity with respect to the amplifier""s input stage or apply an alternative switching scheme to auto-zero the amplifier prior to signal amplification. The problem with both of these techniques is the requirement for switching at the input of the amplifier. In order to achieve switching in a monolithic amplifier, solid state switches must be employed that result in both charge injection and parasitic leakage currents that are often greater than the input transistor leakage. The use of mechanical switches, e.g., reed relays, results in severe bandwidth limitations, finite amplifier lifetime due to switch wear, and physically awkward devices.
These problems with practical modulation of the input signal can be addressed with a class of circuits known as parametric amplifiers. Parametric amplifiers operate by using the variation of some physical parameter to transduce a low-frequency input voltage or current into an upmodulated AC signal. One of the most promising of these techniques is the vibrating capacitor amplifier. In a vibrating capacitor amplifier, the voltage of interest is applied to a capacitor that can be dynamically adjusted. The capacitor magnitude is then modulated, yielding an AC voltage at the amplifier with a frequency above the 1/f corner of the amplifier. The input impedance is defined by the modulation capacitance, yielding, in principle, very high input impedance and low bias currents. This technique has historically provided the most accurate measurements of voltage and charge. A more detailed discussion of vibrating capacitor amplifiers is provided in the article xe2x80x9cDesign of Dynamic Condenser Electrometers,xe2x80x9d The Review Of Scientific Instruments, vol. 18, no. 5, pp. 298-314, May 1947.
The challenge of constructing a vibrating capacitor amplifier is creating a suitable modulation capacitor. In the past, these capacitors have been constructed with the aid of reed relays, mechanically adjustable plates connected to piezoelectrics, or varactors constructed from reverse-biased semiconductor junctions. All of these techniques result in large mechanical structures that are expensive to manufacture in bulk and have limited bandwidth. In addition, the poor physical matching of these structures has effectively prevented the creation of a high-performance differential vibrating capacitor amplifier that can be used for high performance, galvanically isolated, instrumentation amplifiers and electrometer grade operational amplifiers.
A parametric amplifier is provided which has a micromachined variable capacitor structure that includes a drive section, an input sense section, a force feedback section and an input feedback section. The drive section, input sense section, force feedback section and input feedback section each comprise cooperating fixed and movable electrodes. The movable electrodes of each section are connected to a common center structure. One or more electromotive forces generated by one or more voltages applied to the fixed electrodes of the drive section and one or more spring motive forces place the common center structure in motion to oscillate the movable electrodes. The fixed electrodes of the force feedback section provide a velocity signal to an oscillation control circuit. The velocity signal is representative the common center structure""s velocity with respect to the fixed electrodes. The oscillation control circuit generates the one or more electrostatic forces applied to the fixed electrodes of the drive section based upon the velocity signal. A differential input signal applied across the input sense section""s capacitive elements is transduced into a modulated input signal by the oscillation of the movable electrodes. An ac amplifier receives the modulated signal via the common center structure and outputs an amplified modulated input signal. A demodulator receives the amplified, modulated input signal and demodulates it using a reference generated from the velocity signal to output an amplified input signal. When used as an instrumentation amplifier, the output of the demodulator can also be fed back to a secondary set of fixed electrodes of the input feedback section for improved linearity and galvanic isolation. The excellent physical matching of the mechanical structure allows for the creation of differential parametric amplifier topologies, and the small geometry of the micromachined element increases the bandwidth achievable with the device.